A cross-section is a slice through any object which shows its layer structure. In the context of the technical examination of painting it is a minute sample of paint layers.
The sample is mounted in a block of resin and then ground and polished. When it is illuminated under a microscope the pigments present can be identified by their colour and optical properties.
This technique can tell us exactly which pigments were used by an artist. Such analysis is useful for dating a picture, and making matching pigments for retouching damaged areas.